netfs: Add a netfs inode context

Add a netfs_i_context struct that should be included in the network
filesystem's own inode struct wrapper, directly after the VFS's inode
struct, e.g.:

	struct my_inode {
		struct {
			/* These must be contiguous */
			struct inode		vfs_inode;
			struct netfs_i_context	netfs_ctx;
		};
	};

The netfs_i_context struct so far contains a single field for the network
filesystem to use - the cache cookie:

	struct netfs_i_context {
		...
		struct fscache_cookie	*cache;
	};

Three functions are provided to help with this:

 (1) void netfs_i_context_init(struct inode *inode,
			       const struct netfs_request_ops *ops);

     Initialise the netfs context and set the operations.

 (2) struct netfs_i_context *netfs_i_context(struct inode *inode);

     Find the netfs context from the VFS inode.

 (3) struct inode *netfs_inode(struct netfs_i_context *ctx);

     Find the VFS inode from the netfs context.

Changes
=======
ver #4)
 - Fix netfs_is_cache_enabled() to check cookie->cache_priv to see if a
   cache is present[3].
 - Fix netfs_skip_folio_read() to zero out all of the page, not just some
   of it[3].

ver #3)
 - Split out the bit to move ceph cap-getting on readahead into
   ceph_init_request()[1].
 - Stick in a comment to the netfs inode structs indicating the contiguity
   requirements[2].

ver #2)
 - Adjust documentation to match.
 - Use "#if IS_ENABLED()" in netfs_i_cookie(), not "#ifdef".
 - Move the cap check from ceph_readahead() to ceph_init_request() to be
   called from netfslib.
 - Remove ceph_readahead() and use  netfs_readahead() directly instead.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com

Link: https://lore.kernel.org/r/8af0d47f17d89c06bbf602496dd845f2b0bf25b3.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/beaf4f6a6c2575ed489adb14b257253c868f9a5c.camel@kernel.org/ [2]
Link: https://lore.kernel.org/r/3536452.1647421585@warthog.procyon.org.uk/ [3]
Link: https://lore.kernel.org/r/164622984545.3564931.15691742939278418580.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/164678213320.1200972.16807551936267647470.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/164692909854.2099075.9535537286264248057.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/306388.1647595110@warthog.procyon.org.uk/ # v4

1 files changed, 72 insertions, 29 deletions

diff --git a/Documentation/filesystems/netfs_library.rst b/Documentation/filesystems/netfs_library.rst
index 4eb7e7b7b0fc..9c8bc5666b46 100644
--- a/Documentation/filesystems/netfs_library.rst
+++ b/Documentation/filesystems/netfs_library.rst
@@ -7,6 +7,8 @@ Network Filesystem Helper Library
 .. Contents:
 
  - Overview.
+ - Per-inode context.
+   - Inode context helper functions.
  - Buffered read helpers.
    - Read helper functions.
    - Read helper structures.
@@ -28,6 +30,69 @@ Note that the library module doesn't link against local caching directly, so
 access must be provided by the netfs.
 
 
+Per-Inode Context
+=================
+
+The network filesystem helper library needs a place to store a bit of state for
+its use on each netfs inode it is helping to manage.  To this end, a context
+structure is defined::
+
+	struct netfs_i_context {
+		const struct netfs_request_ops *ops;
+		struct fscache_cookie	*cache;
+	};
+
+A network filesystem that wants to use netfs lib must place one of these
+directly after the VFS ``struct inode`` it allocates, usually as part of its
+own struct.  This can be done in a way similar to the following::
+
+	struct my_inode {
+		struct {
+			/* These must be contiguous */
+			struct inode		vfs_inode;
+			struct netfs_i_context  netfs_ctx;
+		};
+		...
+	};
+
+This allows netfslib to find its state by simple offset from the inode pointer,
+thereby allowing the netfslib helper functions to be pointed to directly by the
+VFS/VM operation tables.
+
+The structure contains the following fields:
+
+ * ``ops``
+
+   The set of operations provided by the network filesystem to netfslib.
+
+ * ``cache``
+
+   Local caching cookie, or NULL if no caching is enabled.  This field does not
+   exist if fscache is disabled.
+
+
+Inode Context Helper Functions
+------------------------------
+
+To help deal with the per-inode context, a number helper functions are
+provided.  Firstly, a function to perform basic initialisation on a context and
+set the operations table pointer::
+
+	void netfs_i_context_init(struct inode *inode,
+				  const struct netfs_request_ops *ops);
+
+then two functions to cast between the VFS inode structure and the netfs
+context::
+
+	struct netfs_i_context *netfs_i_context(struct inode *inode);
+	struct inode *netfs_inode(struct netfs_i_context *ctx);
+
+and finally, a function to get the cache cookie pointer from the context
+attached to an inode (or NULL if fscache is disabled)::
+
+	struct fscache_cookie *netfs_i_cookie(struct inode *inode);
+
+
 Buffered Read Helpers
 =====================
 
@@ -70,38 +135,22 @@ Read Helper Functions
 
 Three read helpers are provided::
 
-	void netfs_readahead(struct readahead_control *ractl,
-			     const struct netfs_request_ops *ops,
-			     void *netfs_priv);
+	void netfs_readahead(struct readahead_control *ractl);
 	int netfs_readpage(struct file *file,
-			   struct folio *folio,
-			   const struct netfs_request_ops *ops,
-			   void *netfs_priv);
+			   struct page *page);
 	int netfs_write_begin(struct file *file,
 			      struct address_space *mapping,
 			      loff_t pos,
 			      unsigned int len,
 			      unsigned int flags,
 			      struct folio **_folio,
-			      void **_fsdata,
-			      const struct netfs_request_ops *ops,
-			      void *netfs_priv);
-
-Each corresponds to a VM operation, with the addition of a couple of parameters
-for the use of the read helpers:
+			      void **_fsdata);
 
- * ``ops``
-
-   A table of operations through which the helpers can talk to the filesystem.
-
- * ``netfs_priv``
+Each corresponds to a VM address space operation.  These operations use the
+state in the per-inode context.
 
-   Filesystem private data (can be NULL).
-
-Both of these values will be stored into the read request structure.
-
-For ->readahead() and ->readpage(), the network filesystem should just jump
-into the corresponding read helper; whereas for ->write_begin(), it may be a
+For ->readahead() and ->readpage(), the network filesystem just point directly
+at the corresponding read helper; whereas for ->write_begin(), it may be a
 little more complicated as the network filesystem might want to flush
 conflicting writes or track dirty data and needs to put the acquired folio if
 an error occurs after calling the helper.
@@ -246,7 +295,6 @@ through which it can issue requests and negotiate::
 
 	struct netfs_request_ops {
 		void (*init_request)(struct netfs_io_request *rreq, struct file *file);
-		bool (*is_cache_enabled)(struct inode *inode);
 		int (*begin_cache_operation)(struct netfs_io_request *rreq);
 		void (*expand_readahead)(struct netfs_io_request *rreq);
 		bool (*clamp_length)(struct netfs_io_subrequest *subreq);
@@ -265,11 +313,6 @@ The operations are as follows:
    [Optional] This is called to initialise the request structure.  It is given
    the file for reference and can modify the ->netfs_priv value.
 
- * ``is_cache_enabled()``
-
-   [Required] This is called by netfs_write_begin() to ask if the file is being
-   cached.  It should return true if it is being cached and false otherwise.
-
  * ``begin_cache_operation()``
 
    [Optional] This is called to ask the network filesystem to call into the